It's not necessarily their true market value - it's just their value to Microsoft at that particular time.

If the project that they're working on is strategically important, and any disruption is unacceptable, it might be in Microsoft's best interests to pay them well above their market rate to ensure nobody quits. But the same engineers in another part of the company may not have received raises at all.

On the flip side, Google may be willing to pay above market value in order to take valuable assets away from their direct competition in key areas.

Just because one company in one circumstance pays them a certain salary, it doesn't mean that's their true market value, unless they could get the same rate elsewhere if they quit tomorrow.